Add AudioStream update_stop

teensy3/AudioStream.cpp

 	return true;
 }
 
+void AudioStream::update_stop(void)
+{
+	NVIC_DISABLE_IRQ(IRQ_SOFTWARE);
+	update_scheduled = false;
+}
+
 AudioStream * AudioStream::first_update = NULL;
 
 void software_isr(void) // AudioStream::update_all()

teensy3/AudioStream.h

 	audio_block_t * receiveReadOnly(unsigned int index = 0);
 	audio_block_t * receiveWritable(unsigned int index = 0);
 	static bool update_setup(void);
+	static void update_stop(void);
 	static void update_all(void) { NVIC_SET_PENDING(IRQ_SOFTWARE); }
 	friend void software_isr(void);
 	friend class AudioConnection;